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There is less evidence for their existence than for the other two types. Some ultra-luminous X ray source s (ULXs) in nearby galaxies are suspected to be IMBHs, with masses of a hundred to a thousand solar masses. The ULXs are observed in star forming regions (e.g., in starburst galaxy M82, see external links for beautiful pictures of this galaxy), and are seemingly associated with young star clusters which are also observed in these regions.
It is not clear how such a black hole would form. On the one hand, they are too massive to be formed by the collapse of a single star, which is how the stellar black holes are thought to form. On the other hand, their environments lack the extreme conditions (i.e., high density and velocities observed at the centers of galaxies) which seemingly lead to the formation of supermassive black holes. There are two popular formation scenarios for IMBHs. First one is the merging of stellar mass black holes and other compact objects by means of gravitational radiation. The second one is the runaway collision of massive stars in dense stellar clusters and the collapse of the collision product into an IMBH.
In November 2004 a team of astronomers reported the discovery of the first intermediate-mass black hole in our Galaxy, orbiting three light-years from Sagittarius A*. This medium black hole of 1,300 solar masses is within a cluster of seven stars, possibly the remnant of a massive star cluster that has been stripped down by the Galactic Centre.(Nature News)(original article) This observation may add support to the idea that supermassive black holes grow by absorbing nearby smaller black holes and stars.